The genetic sequences of different individuals are identical over the majority of the genome and vary on average only at about one base in every 1000. The most common form of variation is a difference at a single base, known as a single nucleotide polymorphism or SNP. At these SNP positions some portion of the population will have one base while another portion of the population with have a different base. SNPs act as markers to locate regions of a genome that may be associated with a particular phenotype, such as a risk for disease.